1. Field of the Invention
The present invention is directed to a computer tomography apparatus of the type having an x-ray source which emits a fan-shaped x-ray beam, shaped by a primary radiation diaphragm, which is rotated around a measurement field so as to transirradiate a measurement field from different directions, the attenuated radiation being incident on a radiation detector which converts the incident radiation into electrical signals which are supplied to a computer for generating images.
2. Description of the Prior Art
In known computer tomography systems of the above type, for example in computer tomography systems of the third generation having an arcuate detector composed of a row of detector elements which is rotated around the measurement field together with the x-ray source, the thickness of the x-ray beam (i.e., the extent of the x-ray beam in a direction perpendicular to the fan plane and perpendicular to the direction of radiation propagation) is constant during the scanning, i.e., during the rotation. When subjects are examined which have a highly elliptical cross section, for example when scanning the shoulder or hip region of a human patient, measured values are obtained which contain a large amount of noise, which leads to disturbing horizontal artifacts in the image which is reconstructed from those measured values by the computer. These image artifacts are caused by the low number of quanta which are able to reach the detector at certain of the angular projections. In known systems, these artifacts are attenuated by means of appropriate computational algorithms. It is also known to minimize such artifacts by regulating the high-voltage supply (generator power). The extent to which the generator power can be modified, however, is very small.